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Fri Nov 14, 2008 1:00 pm

Can anyone tell me how far the warm air mixer valve is supposed to open on a '70 SB when it heats up?

I've adjusted it per the Bentley and when I heat the thermostat, the valve only opens about 1/16 of and inch on both mixers.

I would have thought the thermostat would have expanded as much as the thermostat on the cooling flaps does and open the valve about 3/8 to 1/2 inchs.

Fri Nov 14, 2008 2:54 pm

I think you're looking at this backwards. The mixing valve opens to allow cool air to mix with the heated air, not the other way around. If it's opening a 1/16th of an inch, then it'll mix cool air with the heated air, to drop the temp of the heated air down a little, so it doesn't melt stuff. :wink:

Fri Nov 14, 2008 4:00 pm

Bobnotch wrote: I think you're looking at this backwards. The mixing valve opens to allow cool air to mix with the heated air, not the other way around. If it's opening a 1/16th of an inch, then it'll mix cool air with the heated air, to drop the temp of the heated air down a little, so it doesn't melt stuff. :wink:

I know it opens to mix the cool air with the warm. My question is, how far is it supposed to open?

If you heat a thermostat that opens the cooling flaps, it will expand about 3/8 to 1/2 inch to move the flaps to the fully open position. I was under the impression that the two thermostats on the warm air mixing boxes are the same part no. as the cooling flap thermostat. If this assumption is correct, the valve should open 3/8 to 1/2 inches, not 1/16 inch.

My concern is that a 1/16 of an inch opening does not allow enough cool air in to mix with the warm air. I have noticed that the heat is excessive and I usually have to turn it way down or completely off pretty soon after I turn it on.

Fri Nov 14, 2008 7:29 pm

Is it really the same part? That'd be nice.

If it is similar to the engine part then it will eventually fail, right? Maybe yours are at the end of their lifetime? Just a guess.

Fri Nov 14, 2008 8:45 pm

Air_Cooled_Nut wrote: Is it really the same part? That'd be nice.

If it is similar to the engine part then it will eventually fail, right? Maybe yours are at the end of their lifetime? Just a guess.

Could be but, if it is the same part, they are supposed to fail in the expanded position.

Fri Nov 14, 2008 11:25 pm

I don't think they're the same. I pulled some of a junk square and they didn't have temps on them. I put them in some water on the stove with a thermometer to see at what temp they opened.

The water ended up boiling and they never opened. So it would seem they don't open until over 212*.

That's too hot for my engine.

Sat Nov 15, 2008 6:50 am

I had the same idea earlier this Summer, and found one of the mixer thermostats stamped 140*.
The engine thermostats are 149-158*F (65-70*C).

Edited to replace misinformation.

Sat Nov 15, 2008 9:24 am

According to Bentley, the engine thermostat opens at 65 to 70*F.

And according to my Blue Bentley, the adjustment for the warm air mixer is: insert thermostat, and screw it on about 2 turns. fit deflector plate. place fresh air feed valve in the adapter and screw it on tightly. screw thermostat down until the valve contacts the mixer housing. screw thermostat down a further 1 to 1 1/2 turns to give a preload of 1 to 1.5 mm. secure thermostat with locknut.
They don't give a temp for the thermostat itself though (I looked in many of the sections), but I'd bet it's gotta be pretty high temp for it to open.